1. Field of the Invention
This invention relates generally to control of brushless DC motors. In particular this invention relates to reduction of torque ripple of brushless DC motors.
2. Description of Related Art
It is known that brushless DC motors exhibit torque variations during operation above zero velocity and which are most pronounced at relatively low speeds and when subject to high loads. Contributors to torque variations include effects related to motor current as well as effects associated with motor construction. It is recognized that DC brushless motors which are driven by stepwise motor current commutation, for example, six step commutation, demonstrate relatively greater torque variation than motors designed for and driven with sinusoidal current energization. However, stepwise commutated current energization is relatively simpler to achieve than synthesis of sinusoidal motor current. Consequently it is desirable to effect reduction of torque variation for stepwise commutated brushless DC motors.
Certain torque variations associated with motor current commutation are described in U.S. Pat. No. 4,782,272. In particular, this reference illustrates in FIG. 9 and the associated description the effects of deviation of current commutation from the point at which torques from successively energized windings are equal. This reference discloses a control for effecting commutation at a point beyond the point where the torque of successively energized windings are equal, resulting in a torque peak.
U.S. Pat. No. 4,868,477 describes a control for energizing windings of a variable reluctance motor to reduce torque ripple in positioning servos. The control of this reference includes storage for data defining the magnitude of current for each motor winding at each rotor position to produce a desired torque. The stored data is based on measured motor performance. The current control method of this reference contemplates the simultaneous energization of winding phases to achieve a desired torque magnitude at a particular position and is not suitable for effecting velocity control.
Motor control techniques for reducing torque variation associated with certain current related effects and effects produced in the absence of motor current in axial field permanent magnet motors are described in U.S. Pat. No. 5,223,775. These control techniques are directed particularly to modifying the shape and magnitude of applied sinusoidal motor current where each winding phase is controlled independently.
The known control techniques do not achieve satisfactory reduction of torque variation for brushless DC motors driven with stepwise commutated motor current while retaining comparatively simple current commutation.